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October 2004

Apatite is the gray, six-sided grain
in the center of this microscopeb
photograph of granite from
Yosemite National Park.
[Photo courtesy of Drs. P. Piccoli and P.Candela, Laboratory of Mineral Deposits Research, Geology Department, University of Maryland.]

"Apatite" is an unusual name for a very common family of minerals that appears in small amounts in all kinds of rocks AND in large amounts in bones. The chemical formula for this mineral family looks complicated: Ca5(PO4)3(F,Cl,OH). But, don't fear--the last three components listed in the formula, separated by commas, only show how the chemical composition of the mineral family can vary.

Any one of these three components may be present alone or in combination with the other two (see the table below). Common apatite found in rocks contains all three components, fluorine (F), chlorine (Cl) and hydroxyl (OH). Bone, however is chiefly hydroxyl-rich, with smaller quantities of fluorine and even smaller amounts of chlorine.

From Apatite in Rocks

Virtually all rocks contain some chlorine, making it the 19th most abundant chemical element in the Earth. If you were to look for chlorine in rocks, you would most often find it locked up in tiny grains of apatite. Apatite is found in small amounts in all kinds of rocks--from those cooled from boiling lava to those formed from compacted grains of sand.

The phosphorous in apatite (notice the "P" in the formula) makes this mineral family an important fertilizer. Plants need phosphorus to grow. As rocks weather--exposed to wind and rain and snow at the Earth's surface--they break down both physically into smaller pieces and chemically into simpler compounds. Phosphorus "sticks around," bonded to soils where plants soak it up to construct cells. Compared to phosphorus which mostly remains in soil, chlorine is much more mobile; it is carried away, dissolved in rain and melting snow and ice, gradually making its way to the oceans. If, over long periods of geologic time, salty ocean water evaporates, chlorine may combine with sodium or other elements to form minerals such as
halite, which is common table salt (NaCl). The chlorine in the salt you sprinkle on your garden salad probably started out in apatite (no pun intended). And the vegetables in that salad are composed of trillions of cells constructed with phosphorous which also started out in apatite!

To Apatite in Bones

The name "apatite" comes from the Greek word apate, which means "deceit" because it is easily confused with other minerals. Apatite is softer than many gemstone minerals that look much like apatite.

The human body contains about 206 bones. Bones give our bodies structure and protect vital organs, including the brain, the heart and the lungs. Although bones feel hard and lifeless, they are alive. Every minute bones form hundred of thousands of new blood cells.

Most of our teeth and bones are made of apatite or an apatite-like mineral. How does the growing human body construct bone? The starting materials come from nature--the mineral apatite is an extremely important source of the chemical elements comprising bone. When you eat a varied diet, you make these and other chemical components available to your body, which is "programmed" to grow bones and all your other organs.

To wrap up, there's a lot of truth in the old saying that "you are what you eat." We can thank apatite for supplying the ingredients for healthy bones. As the French say,

"Bon Appétit!"

Follow-Up Questions:

Chlorine and fluorine are both members of the Halide Family of chemical elements. Find this family on the Periodic Table of the Elements and look up the common characteristics of the family members.

Draw a diagram to show the different paths that phosphorous and chlorine take through nature when apatite weathers.

In the microscope photo of apatite, the grain is six-sided because of a special internal arrangement of atoms in the mineral. Minerals have an internal order that shows up clearly when we see mineral crystals.

Search the Internet for pictures of apatite to see apatite crystals. Some of them will be beautiful enough to be gemstones. How many different colors of apatite can your find? Does this mineral live up to the meaning of its name?

For a list of previous "Chlorine Compound of the Month" features, click
here.